Issue #2832083 by rbayliss, pounard, alexpott, dawehner: ThemeRegistryLoader::findTemplate() breaks Twig_Loader_Filesystem signature

Issue #2665992 by alexpott, klausi, heykarthikwithu, xjm, tstoeckler: @file is not required for classes, interfaces and traits

Issue #2600152 by Cottser, dawehner: Make ThemeRegistryLoader only throw an error when $throw is true

Issue #2291449 by Cottser: Add Twig template inheritance based on the theme registry, enable adding Twig loaders